Isolation and Characterization of Monomeric Human RAD51: A Novel Tool for Investigating Homologous Recombination in Cancer.

DNA repair protein RAD51 is a key player in the homologous recombination pathway. Upon DNA damage, RAD51 is transported into the nucleus by BRCA2, where it can repair DNA double-strand breaks. Due to the structural complexity and dynamics, researchers have not yet clarified the mechanistic details of every step of RAD51 recruitment and DNA repair. RAD51 possesses an intrinsic tendency to form oligomeric structures, which make it challenging to conduct biochemical and biophysical investigations. Here, for the first time, we report on the isolation and characterization of a human monomeric RAD51 recombinant form, obtained through a double mutation, which preserves the protein's integrity and functionality. We investigated different buffers to identify the most suitable condition needed to definitively stabilize the monomer. The monomer of human RAD51 provides the community with a unique biological tool for investigating RAD51-mediated homologous recombination, and paves the way for more reliable structural, mechanistic, and drug discovery studies.

Modular assembly of RWD domains on the Mis12 complex underlies outer kinetochore organization.

Faithful chromosome segregation is mandatory for cell and organismal viability. Kinetochores, large protein assemblies embedded in centromeric chromatin, establish a mechanical link between chromosomes and spindle microtubules. The KMN network, a conserved 10-subunit kinetochore complex, harbors the microtubule-binding interface. RWD domains in the KMN subunits Spc24 and Spc25 mediate kinetochore targeting of the microtubule-binding subunits by interacting with the Mis12 complex, a KMN subcomplex that tethers directly onto the underlying chromatin layer. Here, we show that Knl1, a KMN subunit involved in mitotic checkpoint signaling, also contains RWD domains that bind the Mis12 complex and that mediate kinetochore targeting of Knl1. By reporting the first 3D electron microscopy structure of the KMN network, we provide a comprehensive framework to interpret how interactions of RWD-containing proteins with the Mis12 complex shape KMN network topology. Our observations unveil a regular pattern in the construction of the outer kinetochore.

Fast native-SAD phasing for routine macromolecular structure determination.

We describe a data collection method that uses a single crystal to solve X-ray structures by native SAD (single-wavelength anomalous diffraction). We solved the structures of 11 real-life examples, including a human membrane protein, a protein-DNA complex and a 266-kDa multiprotein-ligand complex, using this method. The data collection strategy is suitable for routine structure determination and can be implemented at most macromolecular crystallography synchrotron beamlines.

A high-throughput screening identifies MCM chromatin loading inhibitors targeting cells with increased replication origins.

Replication origin assembly is a pivotal step in chromosomal DNA replication. In this process, the ORC complex binds DNA and, together with the CDC6 and CDT1, promotes the loading of the MCM helicase. Chemicals targeting origin assembly might be useful to sensitize highly proliferative cancer cells. However, identifying such compounds is challenging due to the multistage nature of this process. Here, using Xenopus laevis egg extract we set up a high-throughput screening to isolate MCM chromatin loading inhibitors, which led to the identification of NSC-95397 as a powerful inhibitor of replication origin assembly that targets CDC6 protein and promotes its degradation. Using systems developed to test selective drug-induced lethality we show that NSC-95397 triggers cell death both in human cells and Xenopus embryos that have higher proliferative ability. These findings demonstrate the effectiveness of molecules disrupting DNA replication processes in targeting hyperproliferating cells, highlighting their potential as anti-cancer molecules.

RAGE engagement by SARS-CoV-2 enables monocyte infection and underlies COVID-19 severity.

The spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has fueled the COVID-19 pandemic with its enduring medical and socioeconomic challenges because of subsequent waves and long-term consequences of great concern. Here, we chart the molecular basis of COVID-19 pathogenesis by analyzing patients' immune responses at single-cell resolution across disease course and severity. This approach confirms cell subpopulation-specific dysregulation in COVID-19 across disease course and severity and identifies a severity-associated activation of the receptor for advanced glycation endproducts (RAGE) pathway in monocytes. In vitro THP1-based experiments indicate that monocytes bind the SARS-CoV-2 S1-receptor binding domain (RBD) via RAGE, pointing to RAGE-Spike interaction enabling monocyte infection. Thus, our results demonstrate that RAGE is a functional receptor of SARS-CoV-2 contributing to COVID-19 severity.

A tetracationic porphyrin with dual anti-prion activity.

Prions are deadly infectious agents made of PrPSc, a misfolded variant of the cellular prion protein (PrPC) which self-propagates by inducing misfolding of native PrPC. PrPSc can adopt different pathogenic conformations (prion strains), which can be resistant to potential drugs, or acquire drug resistance, hampering the development of effective therapies. We identified Zn(II)-BnPyP, a tetracationic porphyrin that binds to distinct domains of native PrPC, eliciting a dual anti-prion effect. Zn(II)-BnPyP binding to a C-terminal pocket destabilizes the native PrPC fold, hindering conversion to PrPSc; Zn(II)-BnPyP binding to the flexible N-terminal tail disrupts N- to C-terminal interactions, triggering PrPC endocytosis and lysosomal degradation, thus reducing the substrate for PrPSc generation. Zn(II)-BnPyP inhibits propagation of different prion strains in vitro, in neuronal cells and organotypic brain cultures. These results identify a PrPC-targeting compound with an unprecedented dual mechanism of action which might be exploited to achieve anti-prion effects without engendering drug resistance.

Erratum: A tetracationic porphyrin with dual anti-prion activity.

[This corrects the article DOI: 10.1016/j.isci.2023.107480.].

Drosophila TNFRs Grindelwald and Wengen bind Eiger with different affinities and promote distinct cellular functions.

The Drosophila tumour necrosis factor (TNF) ligand-receptor system consists of a unique ligand, Eiger (Egr), and two receptors, Grindelwald (Grnd) and Wengen (Wgn), and therefore provides a simple system for exploring the interplay between ligand and receptors, and the requirement for Grnd and Wgn in TNF/Egr-mediated processes. Here, we report the crystallographic structure of the extracellular domain (ECD) of Grnd in complex with Egr, a high-affinity hetero-hexameric assembly reminiscent of human TNF:TNFR complexes. We show that ectopic expression of Egr results in internalisation of Egr:Grnd complexes in vesicles, a step preceding and strictly required for Egr-induced apoptosis. We further demonstrate that Wgn binds Egr with much reduced affinity and is localised in intracellular vesicles that are distinct from those containing Egr:Grnd complexes. Altogether, our data provide insight into ligand-mediated activation of Grnd and suggest that distinct affinities of TNF ligands for their receptors promote different and non-redundant cellular functions.

Organizational Principles of the NuMA-Dynein Interaction Interface and Implications for Mitotic Spindle Functions.

Mitotic progression is orchestrated by the microtubule-based motor dynein, which sustains all mitotic spindle functions. During cell division, cytoplasmic dynein acts with the high-molecular-weight complex dynactin and nuclear mitotic apparatus (NuMA) to organize and position the spindle. Here, we analyze the interaction interface between NuMA and the light intermediate chain (LIC) of eukaryotic dynein. Structural studies show that NuMA contains a hook domain contacting directly LIC1 and LIC2 chains through a conserved hydrophobic patch shared among other Hook adaptors. In addition, we identify a LIC-binding motif within the coiled-coil region of NuMA that is homologous to CC1-boxes. Analysis of mitotic cells revealed that both LIC-binding sites of NuMA are essential for correct spindle placement and cell division. Collectively, our evidence depicts NuMA as the dynein-activating adaptor acting in the mitotic processes of spindle organization and positioning.

Persistence of Anti-SARS-CoV-2 Antibodies in Non-Hospitalized COVID-19 Convalescent Health Care Workers.

Although antibody response to SARS-CoV-2 can be detected early during the infection, several outstanding questions remain to be addressed regarding the magnitude and persistence of antibody titer against different viral proteins and their correlation with the strength of the immune response. An ELISA assay has been developed by expressing and purifying the recombinant SARS-CoV-2 Spike Receptor Binding Domain (RBD), Soluble Ectodomain (Spike), and full length Nucleocapsid protein (N). Sera from healthcare workers affected by non-severe COVID-19 were longitudinally collected over four weeks, and compared to sera from patients hospitalized in Intensive Care Units (ICU) and SARS-CoV-2-negative subjects for the presence of IgM, IgG and IgA antibodies as well as soluble pro-inflammatory mediators in the sera. Non-hospitalized subjects showed lower antibody titers and blood pro-inflammatory cytokine profiles as compared to patients in Intensive Care Units (ICU), irrespective of the antibodies tested. Noteworthy, in non-severe COVID-19 infections, antibody titers against RBD and Spike, but not against the N protein, as well as pro-inflammatory cytokines decreased within a month after viral clearance. Thus, rapid decline in antibody titers and in pro-inflammatory cytokines may be a common feature of non-severe SARS-CoV-2 infection, suggesting that antibody-mediated protection against re-infection with SARS-CoV-2 is of short duration. These results suggest caution in using serological testing to estimate the prevalence of SARS-CoV-2 infection in the general population.

Discovery of Reversible Inhibitors of KDM1A Efficacious in Acute Myeloid Leukemia Models.

Lysine-specific demethylase 1 (LSD1 or KDM1A) is a FAD-dependent enzyme that acts as a transcription corepressor or coactivator by regulating the methylation status of histone H3 lysines K4 and K9, respectively. KDM1A represents an attractive target for cancer therapy. While, in the past, the main medicinal chemistry strategy toward KDM1A inhibition was based on the optimization of ligands that irreversibly bind the FAD cofactor within the enzyme catalytic site, we and others have also identified reversible inhibitors. Herein we reported the discovery of 5-imidazolylthieno[3,2-b]pyrroles, a new series of KDM1A inhibitors endowed with picomolar inhibitory potency, active in cells and efficacious after oral administration in murine leukemia models.

Hexameric NuMA:LGN structures promote multivalent interactions required for planar epithelial divisions.

Cortical force generators connect epithelial polarity sites with astral microtubules, allowing dynein movement to orient the mitotic spindle as astral microtubules depolymerize. Complexes of the LGN and NuMA proteins, fundamental components of force generators, are recruited to the cortex by Gαi-subunits of heterotrimeric G-proteins. They associate with dynein/dynactin and activate the motor activity pulling on astral microtubules. The architecture of cortical force generators is unknown. Here we report the crystal structure of NuMA:LGN hetero-hexamers, and unveil their role in promoting the assembly of active cortical dynein/dynactin motors that are required in orchestrating oriented divisions in polarized cells. Our work elucidates the basis for the structural organization of essential spindle orientation motors.

A Numb-Mdm2 fuzzy complex reveals an isoform-specific involvement of Numb in breast cancer.

Numb functions as an oncosuppressor by inhibiting Notch signaling and stabilizing p53. This latter effect depends on the interaction of Numb with Mdm2, the E3 ligase that ubiquitinates p53 and commits it to degradation. In breast cancer (BC), loss of Numb results in a reduction of p53-mediated responses including sensitivity to genotoxic drugs and maintenance of homeostasis in the stem cell compartment. In this study, we show that the Numb-Mdm2 interaction represents a fuzzy complex mediated by a short Numb sequence encompassing its alternatively spliced exon 3 (Ex3), which is necessary and sufficient to inhibit Mdm2 and prevent p53 degradation. Alterations in the Numb splicing pattern are critical in BC as shown by increased chemoresistance of tumors displaying reduced levels of Ex3-containing isoforms, an effect that could be mechanistically linked to diminished p53 levels. A reduced level of Ex3-less Numb isoforms independently predicts poor outcome in BCs harboring wild-type p53. Thus, we have uncovered an important mechanism of chemoresistance and progression in p53-competent BCs.

Thieno[3,2-b]pyrrole-5-carboxamides as New Reversible Inhibitors of Histone Lysine Demethylase KDM1A/LSD1. Part 2: Structure-Based Drug Design and Structure-Activity Relationship.

The balance of methylation levels at histone H3 lysine 4 (H3K4) is regulated by KDM1A (LSD1). KDM1A is overexpressed in several tumor types, thus representing an emerging target for the development of novel cancer therapeutics. We have previously described ( Part 1, DOI 10.1021.acs.jmedchem.6b01018 ) the identification of thieno[3,2-b]pyrrole-5-carboxamides as novel reversible inhibitors of KDM1A, whose preliminary exploration resulted in compound 2 with biochemical IC50 = 160 nM. We now report the structure-guided optimization of this chemical series based on multiple ligand/KDM1A-CoRest cocrystal structures, which led to several extremely potent inhibitors. In particular, compounds 46, 49, and 50 showed single-digit nanomolar IC50 values for in vitro inhibition of KDM1A, with high selectivity in secondary assays. In THP-1 cells, these compounds transcriptionally affected the expression of genes regulated by KDM1A such as CD14, CD11b, and CD86. Moreover, 49 and 50 showed a remarkable anticlonogenic cell growth effect on MLL-AF9 human leukemia cells.

Thieno[3,2-b]pyrrole-5-carboxamides as New Reversible Inhibitors of Histone Lysine Demethylase KDM1A/LSD1. Part 1: High-Throughput Screening and Preliminary Exploration.

Lysine specific demethylase 1 KDM1A (LSD1) regulates histone methylation and it is increasingly recognized as a potential therapeutic target in oncology. We report on a high-throughput screening campaign performed on KDM1A/CoREST, using a time-resolved fluorescence resonance energy transfer (TR-FRET) technology, to identify reversible inhibitors. The screening led to 115 hits for which we determined biochemical IC50, thus identifying four chemical series. After data analysis, we have prioritized the chemical series of N-phenyl-4H-thieno[3, 2-b]pyrrole-5-carboxamide for which we obtained X-ray structures of the most potent hit (compound 19, IC50 = 2.9 µM) in complex with the enzyme. Initial expansion of this chemical class, both modifying core structure and decorating benzamide moiety, was directed toward the definition of the moieties responsible for the interaction with the enzyme. Preliminary optimization led to compound 90, which inhibited the enzyme with a submicromolar IC50 (0.162 µM), capable of inhibiting the target in cells.

Crystallization and X-ray diffraction of LGN in complex with the actin-binding protein afadin.

Asymmetric stem-cell divisions are fundamental for morphogenesis and tissue homeostasis. They rely on the coordination between cortical polarity and the orientation of the mitotic spindle, which is orchestrated by microtubule pulling motors recruited at the cortex by NuMA-LGN-Gαi complexes. LGN has emerged as a central component of the spindle-orientation pathway that is conserved throughout species. Its domain structure consists of an N-terminal TPR domain associating with NuMA, followed by four GoLoco motifs binding to Gαi subunits. The LGN(TPR) region is also involved in interactions with other membrane-associated proteins ensuring the correct cortical localization of microtubule motors, among which is the junctional protein afadin. To investigate the architecture of LGN(TPR) in complex with afadin, a chimeric fusion protein with a native linker derived from the region of afadin upstream of the LGN-binding domain was generated. The fusion protein behaves as a globular monomer in solution and readily crystallizes in the presence of sulfate-containing reservoirs. The crystals diffracted to 3.0 Šresolution and belonged to the cubic space group P213, with unit-cell parameter a = 170.3 Å. The structure of the engineered protein revealed that the crystal packing is promoted by the coordination of sulfate ions by residues of the afadin linker region and LGN(TPR).

Structure of a ubiquitin-loaded HECT ligase reveals the molecular basis for catalytic priming.

Homologous to E6-AP C terminus (HECT) E3 ligases recognize and directly catalyze ligation of ubiquitin (Ub) to their substrates. Molecular details of this process remain unknown. We report the first structure, to our knowledge, of a Ub-loaded E3, the human neural precursor cell-expressed developmentally downregulated protein 4 (Nedd4). The HECT(Nedd4)~Ub transitory intermediate provides a structural basis for the proposed sequential addition mechanism. The donor Ub, transferred from the E2, is bound to the Nedd4 C lobe with its C-terminal tail locked in an extended conformation, primed for catalysis. We provide evidence that the Nedd4-family members are Lys63-specific enzymes whose catalysis is mediated by an essential C-terminal acidic residue.

Symmetric dimethylation of H3R2 is a newly identified histone mark that supports euchromatin maintenance.

The asymmetric dimethylation of histone H3 arginine 2 (H3R2me2a) acts as a repressive mark that antagonizes trimethylation of H3 lysine 4. Here we report that H3R2 is also symmetrically dimethylated (H3R2me2s) by PRMT5 and PRMT7 and present in euchromatic regions. Profiling of H3-tail interactors by SILAC MS revealed that H3R2me2s excludes binding of RBBP7, a central component of co-repressor complexes Sin3a, NURD and PRC2. Conversely H3R2me2s enhances binding of WDR5, a common component of the coactivator complexes MLL, SET1A, SET1B, NLS1 and ATAC. The interaction of histone H3 with WDR5 distinguishes H3R2me2s from H3R2me2a, which impedes the recruitment of WDR5 to chromatin. The crystallographic structure of WDR5 and the H3R2me2s peptide elucidates the molecular determinants of this high affinity interaction. Our findings identify H3R2me2s as a previously unknown mark that keeps genes poised in euchromatin for transcriptional activation upon cell-cycle withdrawal and differentiation in human cells.